The present invention relates to a focus servo lead-in method, and more particularly to a focus servo lead-in method and a reproducing device which reproduces information from an optical information recording disk (hereinafter referred to simply as a "disk").
In an optical disk reproducing device, a beam of light converged by an objective lens is projected on the recording surface of the disk. The reading of recorded information is carried out by means of the reflected light (or transmitted light) of the light beam. This requires a precise convergence of the light beam on the recording surface at all times. Thus, a focus servo device must control the position of the objective lens in its optical axis direction.
In such a servo device, if the distance between the objective lens and the recording surface for exact convergence of the light beam on the recording surface of the disk is called D.sub.0, the signal level of a focus error signal is at a reference level (e.g., zero level) when the objective lens is at that focused position. The focus error signal varies depending upon the degree of separation when the distance between the objective lens and the reading surface is more or less than D.sub.0, with the signal being generated with corresponding polarity based on the light beam passed to the recording surface. Accordingly, the characteristic of the focus error signal for the separation distance between the objective lens and the recording surface is that of a so-called S-shaped curve with its center at the point corresponding to the distance D.sub.0. By controlling an actuator for driving the objective lens in the optical direction in response to the signal level of the focus error signal, it becomes possible to always precisely converge the light beam on the recording surface even for a warped disk or the like by enabling the objective lens to move in the direction perpendicular to the recording surface.
As shown in the timing diagram of FIG. 3, the objective lens is generally placed at an initial position close to the recording surface before the start of information reading, so that the lead-in of focus servo is started from that initial position. In the past, for lead-in of the focus servo, the focus servo loop was first set to an open state. A lead-in start command would then be issued when the system was initialized, and the objective lens would be driven from its initial position in a direction away from the recording surface of the disk for a predetermined duration of time T. At time t.sub.1, the direction in which the objective lens was moving would be reversed so that the lens approached the recording surface with a predetermined velocity. When the signal level of the focus error signal shown in FIG. 3(b) attained the reference level at time t.sub.3, the servo loop was closed.
The constant velocity in the system described above varied with the voltage of the power supply, sensitivity of the servo loop, and the loop gain, and the lead-in would be set to be securely locked in when the servo loop was closed. Therefore, the minimum in-focusing time between the drive starting time t.sub.0 and the loop closing time t.sub.3 was also necessarily fixed. Ordinarily, the in-focusing time was on the order of several hundred milliseconds. When a short starting time was required, such as when using a recording medium like a CD-ROM, it was difficult to reduce the starting time because of the long in-focusing time required.